This paper defines the preponderance of the evidence standard and establishes it as a benchmark for cases where deterrence is the dominant concern. The standard is shown to be optimal under the following assumptions: only efficiency matters (not fairness); people are risk-neutral; sanctions are socially costless; and investigations may take place even if no violation of the law has occurred.Concerning the definition of preponderance of the evidence a distinction is made between standards based on probability of guilt and standards based on the evidence. The latter does not include ex-ante information concerning the offender's type, and should hence not be associated with a probability of guilt.